Nitrobenzene (C6H5NO2) is an aromatic compound that is widely used as a solvent and as a mild oxidizing agent. In the chemical industry it is primarily used in the production of aniline and aniline derivatives, such as methylene diphenyl diisocyanate (MDI); however, it also finds use in the manufacture of other chemicals, rubber, pesticides, dyes and pharmaceuticals. In the pharmaceutical industry nitrobenzene is used, for instance, in the production of the analgesic paracetamol (acetaminophen).
The most common reagent used in conventional methods for preparing nitrobenzene is nitric acid or a mixed acid, typically, a mixed solution of concentrated nitric acid and concentrated sulfuric acid, oleum or fuming sulfuric acid. The process generally includes initially forming a nitronium ion, NO2+ by the reaction of nitric acid with concentrated sulfuric acid, as follows:HNO3+2H2SO4→NO2++H3O++2HSO4−The nitronium ion then reacts with benzene to form nitrobenzene, according to the following reaction:

This mixture of acids forms an electrophile which reacts with the benzene in an aromatic electrophilic substitution reaction known as a nitration reaction. The nitric acid is protonated by the sulfuric acid to form H2NO3+, which then loses water to form NO2+. The concentrated sulfuric acid has a high affinity for the water, which facilitates the reaction. Following formation of nitrobenzene, it may be separated from spent and unspent acids by drawing off the sulfuric acid, and returning it to the benzene nitration process as concentrated sulfuric acid.
Such processes are strongly influenced by a number of factors, such as temperatures, and pressures. Appropriate selection of these factors is important, as selection influences the reaction trend, the reaction velocity, and the overall technical and economic balance of the production, in terms of yield, and catalyst consumption, if applicable, and also from the point of view of the intricacy and costs of installation and upkeep. These costs are influenced, for example, by the pressures attained, the consumption of thermal energy for reaching desired temperatures, and the intricacy and the number of component parts of the installation. For instance, in many applications it is desirable to enhance the degree of conversion of benzene. While increasing the reaction pressure may increase reaction rate, it also increases wear of the materials constituting the reactors, the pipings, and the mechanical parts of the plant, as well as any ancillary devices. Most existing processes and production facilities for making nitrobenzene are subject to a variety of constraints such as product yield, plant size, energy consumption and mass flow limitations. Accordingly, there is continuing interest in improving the ways that nitrobenzene is produced.